Up to now, a communication system that performs a wireless communication between a vehicle-mounted device mounted in a vehicle and a mobile device carried by a user has been known (for example, refer to Patent Literature 1). The communication system is applied to an electronic key system that permits locking and unlocking operation of, for example, a vehicle door. As the electronic key system, there is an RKE system (RKE: remote keyless entry) in which an operation signal is transmitted from the mobile device to the vehicle-mounted device on the basis of the operation of a lock button or an unlock button provided in the mobile device, and the vehicle-mounted device locks or unlocks the door on the basis of the operation signal.
In addition to the RKE system, there is an electronic key system (called PEPS system (PEPS: passive entry & passive starting), a smart entry system, and so on) in which a response signal is transmitted from a mobile device in response to a request signal from the vehicle-mounted device, and the vehicle-mounted device locks and unlocks the door on the basis of the response signal. Patent Literature 1 discloses the electronic key system in which a transmission period or a frequency band of search radio waves (request signals) that are periodically transmitted from the vehicle-mounted device is changed suitably in order to detect that the user carrying the mobile device approaches the vehicle. According to the above system, a power consumption can be suppressed, and keyless operation of the vehicle is performed without a delay of authentication of an ID code and with certainty and appropriate timing.
However, in the technique of Patent Literature 1, since an optimum control is not considered for the transmission and reception of the response signal to be transmitted from the mobile device to the vehicle-mounted device, a responsiveness of the response signal may be improved.